Miniature microphones, such as those used in hearing aids, convert acoustical sound waves into an electrical signal which is processed (e.g., amplified) and sent to a receiver of the hearing aid. The receiver then converts the processed signal to acoustical sound waves that are broadcast towards the eardrum.
In one typical microphone, a moveable diaphragm and a rigid backplate, often collectively referred to as an electret assembly, convert the sound waves into the audio signal. The diaphragm is usually a polymer, such as mylar, with a metallic coating. The backplate usually contains a charged dielectric material, such as Teflon, laminated on a metallic carrier which is used for conducting the signal from the electret assembly to other circuitry that processes the signal.
The backplate and diaphragm are separated by a spacer that contacts these two structures at their peripheries. Because the dimensions of the spacer are known, the distance between the diaphragm and the backplate at their peripheries is known. When the incoming sound causes the diaphragm to move relative to the charged backplate, a signal is developed that corresponds to the incoming sound. If the charge on the backplate changes, the signal changes.
Because the charge on the backplate is induced in the material of the backplate, usually by corona charging, the charge can slowly decay over time. Additionally, foreign material that comes in contact with the charged layer can accelerate the charge degradation as the foreign material may have a charge that affects the charged layer. For example, the charge can be reduced by condensed vapor or dirt contacting the charged layer of the backplate. Second, the conductive material on the conductive member that is in contact with the charged layer can release positive (i.e., holes) or negative (i.e., electrons) charges into the charged layer, causing a change in the charge. This effect is at least, in part, due to the surface topography of the conductive layer. Furthermore, extreme ambient conditions, such as temperature and humidity, and light (especially UV light) can also cause a change in the charge.
A need exists for a microphone that has a backplate that is less sensitive to extreme environmental conditions and the infiltration of charges caused by exposure to foreign materials, thereby yielding a more stable charge over the operating life of the backplate.